If process startup time is significant. Note that shell has a lower startup cost than Perl.

I'd not include that item. It sounds very much like premature micro-optimization. Recently someone was ranting about 'sed' being better in some cases than Perl because it starts up faster. My testing (on two very different systems) showed Perl starting up about twice as fast as 'sed' and both starting fast enough that the difference was unlikely to be noticed. I think you'd be hard pressed to properly document how little your shell script can do and still keep this "advantage" over a Perl script.